Diesel engines have several advantages. Compared to traditional spark ignition engines, they generally have lower fuel consumption and a long service life. However, diesel engines have the disadvantage of producing emissions with significant concentrations of nitrogen oxides (NO.sub.x) and particulates. The latter are generally manifested in the form of a black exhaust smoke or soot. Diesel engines also have a reputation for exhibiting significant noise and vibration during operation.
In recent years, the generation of NO.sub.x by internal combustion engines has been the focus of increasing government regulation. It is believed that nitrogen oxides contribute to tropospheric ozone, a health hazard, and also undergo a process known as photochemical smog formation in the presence of sunlight and hydrocarbons. Nitrogen oxides significantly contribute to the formation of acid rain have been implicated as contributing to the undesirable warming of the atmosphere or greenhouse effect. It is anticipated that allowable NO.sub.x emissions from motor vehicles will be significantly decreased in the coming years.
Unfortunately, however, attempts to reduce NO.sub.x emissions in diesel engines have generally resulted in only inadequate NO.sub.x reduction with a concurrent increase in particulate emissions. While NO.sub.x emissions in the exhaust streams of spark ignition engines have been reduced with the use of standard redox catalysts, the lean exhaust streams inherent in diesel engines preclude such use.
Other prior art techniques directed toward the reduction of NO.sub.x in combustion engine exhaust streams are unsuitable for use in motor vehicles. For example, U.S. Pat. No. 4,973,399 discloses a process in which nitrogen oxides contained in a flu gas resulting from the catalytic cracking of hydrocarbon were reduced with the use of a highly siliceous crystalline zeolite having a particular crystal structure, a silica to alumina ratio of 20 to 100, the zeolite containing both copper and rare earth cations. However, this process is dependent upon the use of a circulating inventory of solid acidic cracking catalysts and is inapplicable to the structure and cost restrictions inherent in motor vehicle design.
Several prior art patents attempt to decrease noxious combustion emissions by relying upon the reaction of ammonia with nitrogen oxides to produce nitrogen and water. U.S. Pat. No. 4,978,514 discloses a process for reducing nitrogen oxides in combustion outflow. The process requires introduction of ammonia or an ammonia precursor into the NO.sub.x containing effluent stream and the subsequent passing of the treated stream over a catalyst bed capable of reducing NO.sub.x.
Similarly, U.S. Pat. No. 4,744,926 relies upon the addition of NO.sub.x to an ammonia containing stream prior to the stream's entry into a combustion zone and the subsequent production of nitrogen and water. Several characteristics make these processes inapplicable to use in commercial diesel powered vehicles. The on-board transport of a noxious chemical such as ammonia or an ammonia precursor and its continuous addition to an auto exhaust stream represent a potentially hazardous situation. In addition, these are complex processes which require careful regulation of experimental variables such as time, temperature and reactant concentration.
U.S. Pat. No. 4,886,650 proposes using the sublimation product of cyanuric acid to lower the NO.sub.x content of an exhaust gas stream. However, substantial amounts of this material would be required over the useful life of a diesel vehicle.
Thus, the prior art has failed to address the problem of diesel engine exhaust streams containing unacceptable levels of NO.sub.x and particulates. More specifically, the prior art has failed to reduce the NO.sub.x emission of internal combustion engines and compression ignition engines such as diesel engines without increasing particulate emissions.